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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: configure() -- a new C compiler preprocessor and code injection proposal
Date: Tue, 01 Sep 2015 08:12:35 -0700
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David Brown <david.brown@hesbynett.no> writes:
[...]
>> On Monday, August 31, 2015 at 2:22:12 PM UTC-4, Keith Thompson wrote:
>>> How would you implement offsetof() without using a macro?
>>>
>>> offsetof (defined in <stddef>) takes two arguments, a type name and a
>>> member name.  For example:
>>>
>>>     struct foo {
>>>         int x;
>>>         int y;
>>>     };
>>>
>>>     offsetof(struct foo, x) == 0
>>>     offsetof(struct foo, y) == sizeof (int) (most likely)
[...]
> You would need casts to integer types here:
>
> int lnOffsetX = ((uintptr_t) &foo.x - (uintptr_t) &foo);

No, that would not work portably.  It assumes that uintptr_t
exists, which is not guaranteed (there might not be any unsigned
integer type wide enough to hold a converted pointer value without
loss of information).  And it assumes that arithmetic on uintptr_t
corresponds to arithmetic on pointers, which also is not guaranteed
-- and I've worked on systems where it doesn't.

Casting to char rather than to uintptr_t would work, except that ...

> Of course, that requires the existence of an object "foo" of the struct
> type - while offsetoff works /without/ having any known object of that type.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"